Still, it's not clear if the treatments cause the hike in risk, and the benefits of IVF far outweigh any obstetric dangers, the study's Canadian authors said.

"It is important to remember that the absolute number of women who develop these complications remains quite small, meaning that for most women who cannot conceive naturally, this treatment is a very safe and effective method of becoming pregnant and having a child," said lead researcher Dr. Natalie Dayan. She is director of obstetrical medicine at McGill University Health Center in Montreal.

In the study, Dayan and her colleagues tracked data on nearly 814,000 live births and stillbirths at hospitals in the Canadian province of Ontario between 2006 and 2012.

They compared more than 11,500 women who conceived through infertility treatment with more than 47,500 women who conceived without such therapies. The women who conceived with infertility treatment were typically older, had higher incomes, were more often first-time moms, and carried multiple fetuses.

Rates of severe pregnancy complications -- events such as such as severe bleeding after giving birth, admission to the intensive care unit, and the blood infection sepsis -- were about 31 per 1,000 infertility-treated pregnancies and 22 per 1,000 untreated pregnancies, the research showed.

This higher risk was seen among women who had IVF, but not among those who had other forms of infertility treatment, such as intrauterine insemination or ovulation induction with medication, according to the study published Feb. 4 in the Canadian Medical Association Journal.

Overall, "we found that the women who received infertility treatment, especially in vitro fertilization, were about 40 percent more likely to experience a severe pregnancy complication compared with women who gave birth without any treatment," Dayan said in a journal news release.

Like previous studies, this one found that being older than 40 and being pregnant with twins or triplets is associated with higher rates of serious pregnancy complications, the research team noted. Infertility treatment is common in older women, and pregnancy with multiples is more likely after infertility treatment.

So, "whether the increased risk [for IVF recipients] is a reflection of those who require or choose in vitro fertilization, remains to be determined," the authors wrote.

One U.S. fertility expert agreed.

"It is unclear which the real culprit is," said Dr. Avner Hershlag, who directs reproductive endocrinology at Northwell Health in Manhasset, N.Y. He noted that the study design didn't include information on other medical conditions the women might have had.

Without that data, it "seems more plausible" that certain medical issues, such as obesity, might be the real driving force behind the findings, not the use of IVF itself, Hershlag said.

"Further studies may allow us to clarify these important issues," he said.